Reflective stereoscopic display with first and second liquid crystal cells

ABSTRACT

An apparatus comprising a first cell ( 10   b ), said first cell comprising a plurality of first elements ( 34   b ), said first elements being controllable between a non-reflective state, in which electromagnetic radiation having a first polarization is reflected to a first extent, and a reflective state, in which said electromagnetic radiation having a first polarization is reflected to a second extent, said second extent being greater than said first extent. Said apparatus further comprises a second cell ( 10   a ), superimposed on the first cell, said second cell comprising a plurality of second elements ( 34   a ), said second elements being controllable between a non-reflective state, in which electromagnetic radiation having a second polarization is reflected to a third extent, and a reflective state, in which said electromagnetic radiation having a second polarization is reflected to a fourth extent, said fourth extent being greater than said third extent. Said first and second elements are arranged so that said first polarization is different from said second polarization.

The present invention relates to an apparatus comprising a first cell,said first cell comprising a plurality of first elements, said firstelements being controllable between a non-reflective state, in whichelectromagnetic radiation having a first polarization is reflected to afirst extent, and a reflective state, in which said electromagneticradiation having a first polarization is reflected to a second extent,said second extent being greater than said first extent; and a secondcell, superimposed on the first cell, said second cell comprising aplurality of second elements, said second elements being controllablebetween a non-reflective state, in which electromagnetic radiationhaving a second polarization is reflected to a third extent, and areflective state, in which said electromagnetic radiation having asecond polarization is reflected to a fourth extent, said fourth extentbeing greater than said third extent.

The present invention also relates to a reflective display comprising anapparatus of the aforementioned type and to a portable device comprisingsuch a reflective display.

The present invention also relates to a method of providing varyingbrightness in an apparatus of the aforementioned type.

The present invention finally relates to a method of providing twoimages in a reflective display comprising an apparatus of theaforementioned type.

Evolution has provided many living creatures on our planet with twoeyes, which are spatially separated from each other within the head,thus having two separate, but not necessarily different, fields ofvision. Natural selection has evidently proved that it is verybeneficial if these two separate fields of vision are arranged tolargely overlap each other, thus providing two slightly differentperspective views of the observed surroundings to the brain, which iscapable of combining the provided information and using it in estimatingthe distance to an observed object. The capability of determiningdistances and differences in distances using two eyes is often referredto as stereoscopic vision.

Prior art comprises several so called stereoscopic display devices,which accordingly endeavor to provide users with the sensation of a morerealistic perception of different kinds of presented images such as3D-movies (more formally known as stereoscopic movies). Stereoscopicvision enhances a user's experienced degree of realism in comparisonwith standard, non-stereoscopic display devices, and would be anadvantage in various applications of entertainment, such as movies,games etc., and education, such as flight simulators. Stereoscopicvision would also be an advantage in various other applications, forinstance, in so-called telemedicine wherein a remote medical expert'sstereoscopic perception of a studied object such as a human organ wouldbe an outstanding aid, both in various diagnostic applications andtreatment applications.

The fundamental approach to providing a user with a sensation ofstereoscopic vision, i.e. the sensation of “depth” in an image, consistsof providing two different images, one for each of the two eyes of anobserver, said two different images corresponding to two perspectiveviews, which preferably correspond to the perspective views the observerwould normally acquire, should he or she study the depicted object froma certain distance using both eyes.

A display apparatus is often designed and arranged in such a way thatthe two eyes of an observer are able to see both images, which is whythe current approach to providing the experience of stereoscopic visionnecessitates the provision of eyewear to an observer, wherein theeyewear comprises means which somehow select the image which is intendedfor each eye, so that the appropriate perspective view is presented toeach eye. Another way of describing the current approach is to statethat a single display apparatus presents two separate perspective viewswhich are encoded, and that the user is provided with eyewear whichdecodes the perspective views so that each eye only receives oneintended perspective view.

Prior art comprises apparatuses which are based on the encoding of theperspective views in two colors, for instance red and green,respectively, and the provision of eyewear comprising red and greenfilters, a solution which has the disadvantage of not being able toprovide a stereoscopic experience in color.

Prior art furthermore comprises the alternating provision of twoperspective views and eyewear comprising alternating shutters; anapproach which requires an observer to use bulky, expensive equipmentwith fragile mechanics which will eventually break or that have anoverwhelming weight which renders the usage of such equipment impossibleduring longer periods, and in addition requires computer power andfine-tuned clocks, especially when the two shutters of two observerswould simultaneously observe the same two sequentially projected images.

Finally, prior art further comprises the alternating provision of twoperspective views, which are encoded using polarization, i.e. the lightof the two perspective views have a different polarization, and theprovision of decoding eyewear comprising polarizing means.

A great disadvantage with all prior-art stereoscopic apparatuses is thatthey are only transmissive, which is sometimes an inherent consequenceof the polarization process, wherein the total transmitted intensity isnormally reduced.

A stereoscopic display device has been described in European patentapplication EP 0349692, “Stereoscopic display”, herein incorporated byreference. Said application describes a stereoscopic display which iscapable of displaying monochromatic or colored views of moving,three-dimensional scenes, and which comprises means for sequentiallyprojecting alternate ones of successive pairs of substantiallymonochromatic images corresponding to right-eye and left-eyeperspectives of the scene at a display rate which is sufficient to avoidflicker in the images. A variable polarizer is used to circularlypolarize alternate images in respective opposite senses synchronously attheir projection rate. The images are analyzed by highly transmissiveeyewear comprising at least one pair of oppositely sensed cholestericliquid crystal polarizing means tuned to the particular color wavelengthof the images and disposed, one over each eye, to transmit onlyappropriately polarized images to the corresponding eye.

Said apparatus is associated with several disadvantages, which render itinappropriate in several important applications, where stereoscopicvision would be an advantage. As with other prior-art stereoscopicapparatuses, said apparatus is a transmissive display and thus requiresa backlight unit.

Furthermore, the apparatus comprises means for providing sequentialprojection of images, such means being often noisy and subjected tomechanical strain during operation. Ultimately, these features result inincreased power consumption, mass and volume, which implies that animplementation of such a stereoscopic display in a portable apparatuswould be associated with many disadvantages.

The pictures from said apparatus are furthermore projected sequentially,which involves a regrettable quality reduction, because normal visionimplies the continuous provision of images, i.e. the two eyes receivesimultaneous images of the observed environment.

It is another problem that the images are projected sequentially, evenwhen this is not necessary, as in the case where an identical image,such as a typical 2D-image, is projected, which is a disadvantagebecause unnecessar power consumption and bulky technology is used.

It is thus a great disadvantage that the prior-art apparatuses are notsuitable for the alternating or even simultaneous provision of 2D and 3Dimages, respectively.

Furthermore, the underlying technology deprives the user of apossibility of adjusting the brightness, and since he or she is alreadywearing the compulsory eyewear, brightness reduction cannot be obtainedby conventional methods, for instance by using sunglasses.

Furthermore, the apparatus is not suitable in cases where there arethree intended receivers of three separate images, but is merelyrestricted to provision of the same stereoscopic experience, no matterhow many observers there are.

In the prior art, liquid crystal displays have proved to be suitable forvarious applications which necessitate compactness and a low powerconsumption. A liquid crystal display (LCD) is a flat panel displaydevice having advantages of small bulk, thin thickness and low powerconsumption.

LCDs have been used in connection with portable devices such as mobiletelephones, portable computers, electronic calendars, electronic books,televisions or video game controls and various other office automationequipment and audio/video machinery, etc.

LCDs control an electric field which is applied to a liquid crystalmaterial having a dielectric anisotropy to transmit or shut off light,thereby displaying a picture or an image, all in a fashion known per seas is recognized by those skilled in the art and as will be brieflyexplained. Unlike display devices that generate light internally—such aselectroluminiscence (EL) devices, cathode ray tubes (CRT) andlight-emitting diodes (LED)—LCDs use an external light source.

LCD devices are largely classified into transmissive type devices andreflective type devices, depending on the method of utilizing light.Apart from a liquid crystal panel having a liquid crystal mixtureinjected between two transparent substrates, the transmissive type LCDincludes a backlight unit for supplying light to the liquid crystalpanel. It is, however, very difficult to make a transmissive LCD with athin thickness and a low weight. Moreover, the backlight units oftransmissive LCDs have an excessive power consumption.

In contrast, reflective type LCDs include a reflective liquid crystaldisplay panel that transmits and reflects natural light and peripherallight to and from the display screen without a backlight unit.

Reflective type LCDs are not suitable for any of the prior-artstereoscopic apparatuses, because all of them necessitate a transmissivedisplay, inter alia, because of the reduction in the intensity of lightwhich is inherent in the polarization process.

A basic liquid crystal display can easily be constructed by coating twoseparate thin sheets of a transparent material, such as glass orplastics, with a transparent metal oxide. Preferably, the metal oxide isapplied in the shape of parallel lines on each of the separate sheets,and constitutes the row and column conductors of the LCD. When the twosheets are superimposed with the row conductors perpendicular to thecolumn conductors, the rows and columns form a matrix of pixel elements.The row conductors further serve to set up the voltage across a cell,which is necessary for the orientation translation.

An alignment layer, sometimes referred to as orientation layer, isapplied to each sheet. The alignment layer may have undergone a rubbingprocess resulting in a series of microscopic grooves which are paralleland will assist in aligning the contained liquid crystal molecules in apreferred direction, with their longitudinal axes parallel to thegrooves, which “anchors” the molecules along the alignment layers andhelps the molecules between the alignment layers to twist.

One of the thin sheets is coated with a layer of polymer spacer beads.These beads maintain a uniform gap between the sheets of glass where theliquid crystals are eventually placed. The two glass sheets are thenplaced together and the edges are sealed with epoxy. A corner is leftunsealed so that the liquid crystal material can be injected under avacuum. Once the display has been filled with liquid crystals, thecorner is sealed and polarizers (the transparent layers with lines) areapplied to the exposed glass surfaces.

The display is completed by connecting the row and column conductors tothe driving circuitry which controls the voltage applied to variousareas of the display.

It is an object of the invention to provide a display which is suitablefor the alternating display of 2D and 3D images.

Generally speaking, the essence of the invention is that if a display isbuilt using two liquid crystal cells in a new, specific way, the displaywill have a better performance and several new features. Mostimportantly, the display may function both as a stereoscopic display andas a standard 2D display. Furthermore, the new display design provides amultitude of other features such as enhanced brightness control, thepossible provision of several images to several receivers, and since theconstruction essentially includes two cells, it enables one of them topartially serve as a back-up unit in case the other cell is damaged orfails. Several embodiments are described including those where thestereoscopic experience does not require a user to use eyewear. For thepurpose of illustration, only light at one wavelength is discussed, buta full-color stereoscopic display in a portable device may be acommercial product incorporating the invention.

According to a first aspect, the present invention relates to anapparatus comprising a first cell, said first cell comprising aplurality of first elements, said first elements being controllablebetween a non-reflective state, in which electromagnetic radiationhaving a first polarization is reflected to a first extent, and areflective state, in which said electromagnetic radiation having a firstpolarization is reflected to a second extent, said second extent beinggreater than said first extent; and a second cell, superimposed on thefirst cell, said second cell comprising a plurality of second elements,said second elements being controllable between a non-reflective state,in which electromagnetic radiation having a second polarization isreflected to a third extent, and a reflective state, in which saidelectromagnetic radiation having a second polarization is reflected to afourth extent, said fourth extent being greater than said third extent,wherein said first and second elements are arranged so that said firstpolarization is different from said second polarization.

Preferably, the electromagnetic radiation has a wavelength of between300 nm and 800 nm (i.e. visible light) and said first polarization andsaid second polarization are circular polarizations of oppositehandedness.

The arrangement can be optionally realized by arranging apolarization-altering element, preferably an appropriate halfwave plate,between said first and second cells which, in this case, are arranged toreflect circularly polarized light of the same handedness. Thepolarization-altering element could comprise a lens. The first andsecond cells may optionally be at a distance from the optical element orfrom each other.

The first and second cells are preferably arranged to transmit a firstand a second image to the first and the second eye of an observer. Thewavelengths of the light reflected by the respective two cells do notnecessarily have to be the same. Preferably, said first and second cellsare at least partially made of cholesteric texture liquid crystal(CTLC).

According to a second aspect, the present invention relates to areflective display comprising an apparatus of the aforementioned type.

According to a third aspect, the present invention relates to a portabledevice comprising such a reflective display. Such a portable display ispreferably, but not necessarily, one of a mobile telephone, a portablecomputer, an electronic calendar, an electronic book, a television setor a video game control.

According to a fourth aspect, the present invention relates to a methodof providing varying brightness in an apparatus of the aforementionedtype. This method of providing different levels of brightness can alsobe applied to apparatuses comprising more than two cells.

According to a fifth aspect, the present invention relates to a methodof providing two or more images in a reflective display which comprisesan apparatus of the aforementioned type. Preferably, said method can beused to provide different images to the left and right eyes,respectively, said images preferably being perspective viewscorresponding to the left eye and right eye perspective, respectively,of an observed object or environment. Said method can be preferably usedto provide a possibility of switching between 2D and 3D vision using thesame apparatus.

These and other aspects, features and advantages of the invention areapparent from and will be elucidated with reference to the embodimentsdescribed hereinafter.

FIG. 1 is a schematic side view of a part of a first, preferredembodiment of a liquid crystal display according to the invention.

FIG. 2 is a schematic side view of a second embodiment of a liquidcrystal display according to the invention.

FIG. 3 is a schematic side view of a third embodiment of a liquidcrystal display according to the invention.

FIG. 4 is a schematic block diagram of a typical prior-art arrangementfor controlling and driving an electro-optic display device.

FIG. 5 is a graph illustrating the reflection vs. applied field strengthof a liquid crystal mixture for a predefined wavelength.

FIG. 6 is a graph of the reflection in percentage as a function of thewavelength for illustrating the wavelength dependency of reflectionproperties for three different liquid crystal mixtures.

FIG. 7 illustrates a scenario wherein an observer experiences simulatedstereoscopic vision.

FIG. 8 illustrates another scenario wherein an observer experiencessimulated stereoscopic vision.

FIG. 9 illustrates yet another scenario wherein three observers receivethree different images from the same screen.

The embodiments of the present invention will now be described withreference to FIGS. 1 to 9 of the drawings. Identical elements in thevarious Figures are denoted by the same reference numerals.

FIG. 1 is a schematic side view which illustrates a part of a preferredembodiment of a liquid crystal display according to the invention. Forthe purpose of illustration; several dimensions such as the size of themolecules and the distances between the sheets of glass have beenexaggerated, and the molecular structures of the liquid crystal mixtureshave been simplified.

Two cells 10 a and 10 b, each comprising their own matrix of elements orpixels, are arranged on top of each other. Thin sheets of glass 30 a, 31a, 30 b, 31 b partially enclose each of the two cells as illustrated,from two opposite sides, which sides constitute essentially parallelplanes. Plastic substrates can be used instead of glass in order toreduce the parallax between the layers, because plastic substrates canbe made thinner than glass.

Each cell 10 a, 10 b comprises its own set of column conductors 12 a, 12b and row conductors 14 a, 14 b, which are implemented as indium tinoxide (ITO) lines, arranged on said sheets of glass 30 a, 31 a, 30 b, 31b in accordance with prior-art LCDs.

Alignment layers (also known as orientation layers) 32 a, 33 a, 32 b, 33b, each of which may be alignment layer SE7511L from Nissan ChemicalIndustries, are arranged on the inside of each cell, as indicated in theFigures in order to orient the enclosed liquid crystals 34 a, 34 b in apreferred way.

Spacer balls (not shown), for instance SP-2050 from Sekisui Chemical,and seal material, e.g. XN21-S from Mitsui Chemical, are preferably usedin accordance with prior-art liquid crystal displays to establish auniform spacing between the thin sheets of glass 30 a and 31 a, whichenclose the upper cell 10 a (CTLC cell 1), and the thin sheets of glass30 b and 31 b, which enclose the lower cell 10 b (CTLC cell 2).

Appropriate liquid crystal mixtures (CTLC materials) 34 a, 34 b arearranged in the upper and lower cells, respectively. In this case, 34 ais a liquid crystal mixture of BL87/BL88 10:90 w:w (Merck) for the uppercell 10 a, and 34 b is a liquid crystal mixture of BL87/BL95 (Merck)3:97 w:w (Merck) for the lower cell 10 b, so that the two cells compriseliquid crystal mixtures which have opposite twists, i.e. they willreflect circularly polarized light of opposite handedness.

As will be appreciated by those skilled in the art, the CTLC material isa mixture of species. Basically, two species are necessary: a nematichost and a chiral dope. The handedness of the dope determines thehandedness of the CTLC, and the concentration of the chiral dopedetermines the wavelength (color) of the reflected light. In order tomake a color display, there are basically two possibilities: make pixelswhich reflect different colors or stack cells which reflect differentcolors on top of each other.

It is also possible to change the color of a CTLC-mixture by applying ahigh electrical field perpendicular to the helical axis, using electrodemeans, which would be one perceivable manner to realize a full-color,3D-display using only two layers.

It is perceivable that, in some applications, an isolating layer will berequired between the two cells to prevent crosstalk between the rowand/or column conductors of the two cells, in particular crosstalkbetween the row conductor 14 a and column conductor 12 b. It is alsoimaginable that the lower substrate 31 a of the upper cell 10 a and theupper substrate 30 b of the lower cell 10 b might be implemented as onesubstrate, possibly comprising a shared column and/or row conductor.

FIG. 2 is a schematic side view of an alternative embodiment of a liquidcrystal display according to the invention, wherein two cells 10 a, 10 bwhich comprise CTLC-mixtures 34 a, 34 b are stacked on top of eachother, and wherein an optical element 35 has been introduced between thetwo cells. For the purpose of simplification, the optical element isillustrated at a distance from the upper and lower cells 10 a, 10 b.

The optical element 35 may be a polarization-altering element, such as ahalfwave plate or another suitable optical component which allows theorientation of circularly polarized light to change, either fromleft-handed orientation to right-handed orientation, or vice versa.

Such an arrangement would allow both cells 10 a, 10 b to be filled withthe same liquid crystal mixtures, which may be a liquid crystal mixtureof BL87/BL88 10:90 w:w (Merck) mentioned before or any other suitableliquid crystal mixture.

In the preferred embodiment, described previously with reference to FIG.1, the light reflected by each cell would have the same polarization ifboth cells comprised the same liquid crystal mixture. In the embodimentof FIG. 2, however, the light reflected from one of the cells would passthrough the optical element and thus change its polarization, thusresulting in the transmission of two images, one from each cell 10 a, 10b, using light of two different polarizations.

Alternatively, the optical element could be a lenticular sheet, asdescribed in U.S. Patents U.S. Pat. No. 6,064,424 or U.S. Pat. No.6,118,584, which are herein incorporated by reference.

According to one aspect of the invention, the embodiment described abovewould have the advantage that the light which is reflected from the twocells 10 a and 10 b would be reflected in slightly different directions.Preferably, these directions are arranged to coincide with the left andright eyes, respectively, of an observer, for instance at a distance ofbetween 0 and 50 cm from the display, should the apparatus beimplemented in a small portable apparatus such as a personal digitalassistant (PDA) or a portable telephone, etc. Such an embodiment wouldenable a user to experience stereoscopic vision without eyewear.

According to another aspect of the invention, the embodiment illustratedin FIG. 2 could be implemented as a large-scale display, such as aTV-set for two persons. The angles in which the two cells reflect lightcould be arranged to coincide with the position of a first and a secondobserver, sitting at a distance from each other.

One can also imagine the great potential in a display where only a partof the display has the embodiment as described with reference to FIG. 2.For instance, the lowermost part of a screen could be constructedaccording to the invention, allowing two observers to watch a TV-programand perceive two different subtitles in two different languages, orstock exchange data or news flashes as they are presented on many TVchannels, where the lowermost space of the TV screen is allocated forstock exchange data, news updates, etc.

In addition to the lenticular sheet, the optical element could compriseanother optical element, for instance a halfwave plate as has beenpreviously discussed with reference to FIG. 2.

FIG. 3 is a schematic side view of another alternative embodiment of aliquid crystal display according to the invention, in which in additionto the specification of FIG. 1 the cells are at a certain distance fromeach other, which implies that light can be reflected from slightlydifferent angles from the first and second cells 10 a, 10 b,respectively. The fact that the CTLC-mixtures 34 a, 34 b which areconfined in the cells reflect polarized light of opposite handedness (orare arranged to reflect polarized light of opposite handedness by meansof the optical element 35 as has been described with reference to FIG.2), and the fact that the cells are essentially transparent to the lightwhich the other cell reflects, implies that the light emanating from thefirst and second cells 10 a, 10 b does not interfere with each other.

FIG. 4 shows a schematic diagram of a typical prior-art arrangement forcontrolling and driving an electro-optic display device. In thisarrangement, a liquid crystal display 10 has a matrix of pixels arrangedvertically in columns and horizontally in rows. These pixels are locatedat the intersections of the column conductors 12 and the row conductors14. The column conductors 12 provide analog voltages to the pixels ineach column, whereas the row conductors 14 provide a switching voltageto each associated row, permitting the column voltages to be supplied tothe pixels of that row.

Rows are successively addressed in a prescribed order by means of a rowdecoder 16 which activates successive ones of a plurality of row drivers18.

Column voltages are supplied by column driver circuits which arerealized as track-and-hold circuits. These track-and-hold circuitsreceive a ramp voltage from the output buffer amplifier of adigital-to-analog converter (DAC) 22. The DAC 22 receives successivedigital numbers from a counter 24 that counts pulses produced by a clock25. The count commences either from some minimum number or maximumnumber and increases or decreases steadily until it reaches, at theopposite end of the scale, a maximum or minimum number, respectively.The DAC thus produces an increasing or decreasing ramp signal, inrepetitive cycles, which approximates its digital input.

The output of the counter 24 is also supplied to a number of comparators26, one for each column. This number is then compared in each comparatorwith a digital number representing the desired brightness level of apixel in the associated column. The number representing this brightnesslevel is stored in an associated pixel register 28 during each completecycle of the system.

When the count supplied by the counter 24 is equal to the digital numberstored in a pixel register, the respective comparator 26 produces apulse which is passed to a track-and-hold circuit for that column. Uponreceiving such an enable pulse, an associated column driver stores avoltage equal to the instantaneous output of a ramp generator.

Upon completion of each ramp cycle, the voltages stored in the columndriver circuits are supplied to a pixel in a particular row selected bythe row drivers 18.

Each cell in an apparatus according to the invention could accordinglybe controlled by such a prior-art arrangement for controlling anddriving an electro-optic display device.

FIG. 5 is a graph illustrating the reflection vs. applied field strengthfrom a liquid crystal mixture for a predefined wavelength. Depending onthe strength of the applied field, the molecules in LCD pixels canswitch between light and dark states, or somewhere in between (grayscale). How the molecules respond to a voltage is the importantcharacteristic of this type of display. The electro-distortionalresponse determines the reflection of light through the cell.

FIG. 6 is a graph of the reflection in percentage as a function of thewavelength for illustrating the wavelength dependency of reflectionproperties for three different liquid crystal mixtures, namely: 90%/10%BL088/BL087, 80%/20% BL088/BL087 and 97%/3% BL095/BL087.

As will be appreciated by those skilled in the art, the demonstratedwavelength dependence of the reflected light for different mixtures canbe exploited to construct full-color displays, such as RGB-displays, bycreating a display, alternately filling the pixels with three differentmixtures, each of which reflects essentially red, green and blue light,respectively.

FIG. 7 illustrates a scenario wherein an observer experiences simulatedstereoscopic vision. A display 40 according to the invention is arrangedat a distance 41 from an observer (not shown). The display 40 comprisestwo superimposed liquid crystal cells 10 a and 10 b as has beendescribed previously with reference to FIGS. 1 to 3. Each of these cellsis connected to the necessary electronics. The upper cell 10 a presentsan image 42 a and the lower cell presents an image 42 b as indicated inFIG. 7. To a naked eye, both images will appear on the display. Theimages 42 a, 42 b are, however, coded using polarization, because thetwo cells 10 a, 10 b are arranged to reflect circularly polarized lightof opposite handedness.

The eyewear 43, which for the purpose of illustration is depicted asglasses, is worn by the observer. The left and the right eye of theobserver are observing the screen 30 through polarizing means 44 a and44 b, respectively, which act as filter elements and are each highlytransmissive to circularly polarized light of one handedness, but nottransmissive to circularly polarized light of the opposite handedness.Consequently, only the image 42 a created by the upper cell 10 a isviewed by the left eye of the observer through the filter element 44 a,and only the image 42 b created by the lower cell 10 b is viewed by theright eye of the observer through the filter element 44 b.

When the image 42 a which is transmitted by the upper cell 10 a is aperspective view, corresponding to a left-eye perspective, and the image42 b which is transmitted by the lower cell 10 b is a perspective view,corresponding to a right-eye perspective, the result is thus twoseparate perspective views for the left and the right eye, respectively,and hence the user experiences stereoscopic vision.

It is evident that the two respective perspective views could be anidentical image, which could be experienced by a user no matter whetherhe or she is wearing the eyewear which is compulsory in the stereoscopicfeature.

The described stereoscopic feature of the invention necessitates eyewearcomprising filter elements which may be implemented, for example, asglasses, the lenses of which comprise appropriate polarizing means.These can be made in several ways, for instance, by means of absorbingpolarizing films for LCD, in combination with lambda/2 retardation filmsfor LCD. The orientation of the ordinary and extraordinary axis relativeto the absorption axis of the polarizing film determines whichhandedness is absorbed and which handedness is transmitted. Thesecomponents can be bought at Nitto-Denko or Sumitomo Chemical. A film ofCTLC can be made which reflects light of one circular polarization andtransmits the other one. These films are, however, relatively expensive.

In the preferred embodiment, the eyewear is implemented as glasses, butit is also possible to implement the eyewear as contact lenses.

FIG. 8 illustrates that the embodiments which have been (partially)illustrated in FIGS. 2 and 3 and previously described with reference tothe aforementioned Figures do not require a user to use decoding eyewearto experience stereoscopic vision. This is because the cells 10 a, 10 bof the apparatus are arranged so that the images which are formed by thetwo cells propagate in slightly different directions as indicated in theFigure, resulting in their coincidence with the left and the right eyeof a user at a certain distance.

The Figure illustrates a deviating optical element such as a lenticularsheet 35 arranged between the cells 10 a, 10 b to deviate the lightwhich is reflected from the lower cell 10 b in a direction slightlydifferent from the direction in which the light from the upper cell 10 ais reflected. It is nevertheless also perceivable to imagine a similareffect using the embodiment described with reference to FIG. 3, becausethis would imply that the two cells are observed from slightly differentdistances and thus from different angles. It is also perceivable thatthe deviation in the direction of propagation of one of the cells couldbe established by simply tilting one of the cells slightly as opposed tothe previously described embodiments where cells are superimposed oneach other, with their thin sheets of glass 30 a, 31 a, 30 b, 31 bessentially parallel to each other.

Although the preferred embodiment may be used in a portable device, itis also possible to imagine other scenarios wherein an apparatusaccording to the invention would prove to be useful.

FIG. 9 illustrates a user scenario wherein the apparatus according tothe invention is used to provide several users with different images.

A large liquid crystal display 50 is arranged on a wall, for instance,in the passenger compartment of an aircraft, within view of threepassengers 52 a, 52 b, 52 c who are situated in different parts of theaircraft and face the liquid crystal display. The display comprisesthree different layers (not shown), each reflecting light having acertain wavelength and polarization properties. For the purpose ofillustration, each of the three cells projects one of the images 51 a,51 b and 51 c.

Three decoding elements 53 a, 53 b, 53 c are arranged in front of eachobserver, said decoding elements being arranged to essentially transmitonly light from one of the cells to each observer. Consequently, theobserver 52 a only perceives the image 54 a, the observer 52 b onlyperceives the image 54 b and the observer 52 c only perceives the image54 c as indicated in the Figure.

For the purpose of simplification, the decoding elements are illustratedas screens, but it would also be possible to implement them as eyewearsuch as glasses which have been described previously.

A display according to the invention may additionally comprise abacklight unit, or it may be implemented as a portable or large-scaletransmissive display.

When an apparatus according to the invention is used as a normaldisplay, i.e. as a 2D-display, without glasses, each eye will receivethe light reflected from both layers of the apparatus. This impliesthat, as a display device, the apparatus according to the invention hasseveral advantages over the prior art. Only one cell could be used at atime, and the second cell could be used as a backup unit in case thefirst cell would fail.

As brightness is an important feature in liquid crystal display devices,it would be advantageous to use the display device according to theinvention as a display device which provides a user with the option ofselecting the degree of brightness. This could be practically realizedby instructing the pixels in only one layer to reflect light when a lowbrightness is desired and instructing the pixels in both layers toreflect light when a higher brightness is desired. Since the layersreflect light which is circularly polarized with opposite handedness,there is no interference between the reflected light, thereby resultingin a sharp image.

This method of providing different levels of brightness could beextended to apparatuses comprising more than two superimposed cells.When a lower brightness is desired, a number N of cells (N being atleast one but not equal to the total number of superimposed cells) couldbe manipulated into reflecting light, and when a higher brightness isdesired, a number N+1 of superimposed cells could be manipulated intoreflecting light.

The apparatus according to the present invention may be realized, forexample, as a separate, stand-alone unit, or may alternatively beincluded in, or combined with, a mobile terminal for a telecommunicationnetwork, such as GSM, UMTS, GPS, GPRS or D-AMPS, or another portabledevice of an existing type, such as a Personal Digital Assistant (PDA),palmtop computer, portable computer, electronic calendar, electronicbook, television set or video game control, as well as various otheroffice automation equipment and audio/video machinery, etc.

The invention has mainly been described with reference to severalembodiments. However, embodiments other than the ones disclosed aboveare equally possible within the scope of the invention, as defined bythe appended patent claims. All terms used in the claims are to beinterpreted according to their ordinary meaning in the technical field,unless explicitly defined otherwise. All references to “a/an/the[element, means, component, member, unit, step, etc.]” is to beinterpreted openly as referring to at least one instance of saidelement, means, component, member, unit, step, etc. The steps of themethods described herein do not have to be performed in the exact orderdisclosed, unless explicitly specified.

1. A reflective display apparatus comprising: a first liquid crystalcell, said first liquid crystal cell comprising a plurality of firstfull-color pixel elements configured to produce full-color images, saidfirst pixel elements being controllable between a non-reflective state,in which electromagnetic radiation having a first polarization isreflected to a first extent, and a reflective state, in which saidelectromagnetic radiation having a first polarization is reflected to asecond extent, said second extent being greater than said first extent,wherein the first liquid crystal cell is further configured to reflectelectromagnetic radiation of the first polarization according to a firstperspective view of a full-color image in a first direction; and asecond liquid crystal cell, said second liquid crystal cell comprising aplurality of second full-color pixel elements configured to producefull-color images, said second pixel elements being controllable betweena non-reflective state, in which electromagnetic radiation having asecond polarization is reflected to a third extent, and a reflectivestate, in which said electromagnetic radiation having a secondpolarization is reflected to a fourth extent, said fourth extent beinggreater than said third extent, wherein the second liquid crystal cellis further configured to reflect electromagnetic radiation of the secondpolarization according to a second perspective view of the full-colorimage in a second direction, the first perspective view being separatefrom the second perspective view, and the first direction beingdifferent from the second direction, further characterized in that saidfirst and second liquid crystal cells are configured so that said firstpolarization is different from said second polarization.
 2. An apparatusaccording to claim 1, wherein the electromagnetic radiation has awavelength of between 300 nm and 800 nm.
 3. An apparatus according toclaim 1, wherein said first polarization and said second polarizationare circular polarizations of opposite handedness.
 4. An apparatusaccording to claim 1, wherein said first and second liquid crystal cellsare configured so that said first polarization is different from saidsecond polarization via a polarization-altering element arranged betweensaid first and second liquid crystal cells.
 5. An apparatus according toclaim 4, wherein said polarization-altering element is a halfwave plate.6. An apparatus according to claim 1, further wherein said first andsecond liquid crystal cells are arranged to transit the firstperspective view of the full-color image alone and the secondperspective view of the full-color image alone to a first eye and asecond eye, respectively, of an observer.
 7. An apparatus according toclaim 1, wherein at least one of said first and second cells is at leastpartially made of cholesteric texture liquid crystal (CTLC).
 8. Aportable device comprising a reflective display according to claim
 1. 9.A portable device according to claim 8, wherein said device is one of amobile telephone, a portable computer, an electronic calendar, anelectronic book, a television set or a video game control.
 10. A methodof providing two separate perspective views of an image in a reflectivedisplay according to claim 1, wherein said method comprises the stepsof: providing at least two separate filter elements, (i) a first of saidtwo filter elements being capable of transmitting electromagneticradiation having said first polarization and not transmittingelectromagnetic radiation having said second polarization, and (ii) asecond of said two filter elements being capable of transmittingelectromagnetic radiation having said second polarization and nottransmitting electromagnetic radiation having said first polarization,arranging the first filter element between the reflective display and anintended receiver of the first perspective view of the full-color imageproduced by the first full-color pixel elements of the first liquidcrystal cell, wherein the intended receiver of the first perspectiveview perceives only the first perspective view of the full-color image,and arranging the second filter element, separately from the firstfilter element, between the reflective display and an intended receiverof the second perspective view of the full-color image produced by thesecond full-color pixel elements of the second liquid crystal cell,wherein the intended receiver of the second perspective view perceivesonly the second perspective view of the full-color image.
 11. A methodaccording to claim 10, wherein the first and second filter elements arearranged in front of the left and the right eye, respectively, of anobserver.
 12. A method according to claim 10, wherein said first andsecond perspective views create a 3D sensation when observed.